4.0.1 i386 wedged

I don't know if anyone recalls enough 4.x to say anything useful here,
but if anyone does and cares to comment....
The machine: 4.0.1 i386. Two CPUs. (Kernel has MULTIPROCESSOR,
MPBIOS, and APM_NO_IDLE turned on.)
NFS-mount a filesystem with a big (half-terabyte) file in it. vnconfig
that onto vnd0. Mount /dev/vnd0d. Write stuff to it.
Machine locks up. Responsive to ping, but userland is totally wedged,
doesn't even respond to RETURN on the console. Break into ddb and do
ps and find pagedaemon is waiting on emergva. It appears to be
deadlocked against itself; my impression from the stack trace is that
it's trying to page something out and finds itself wanting to page
something in to do so.
Is this a case of "don't do that, then", or should this work and I just
need to track down a bug?
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